The invention relates to a method for anodizing aluminum materials and aluminized parts in aqueous alkaline anodizing electrolytes containing phosphate ions.
Untreated aluminum materials and aluminum-coated parts are not resistant enough to oxidation and wear for many applications, although a resistant oxide coating, which is stable in the pH-range of 5 to 9, is produced in air. This oxide coating, however, is too thin for many technical applications. Therefore, the layer can be reinforced in anodizing electrolytes.
It is known to anodize aluminum anodically, using diluted sulfuric acid (d-c sulfuric acid process), diluted oxalic acid (d-c oxalic acid process) or also in a sulfuric acid/oxalic acid bath. While these processes can be used for aluminum materials, destruction of the material or at least pitting which is not tolerable, occurs in aluminized parts if an aluminum cover is not provided on all sides.
A "method for bright anodizing of aluminum" is described in U.S. Pat. No. 4,188,270. According to U.S. Pat. No. 4,188,270 an alkaline bath with sodium phosphate as the main component is used for the preparation of bright aluminum oxide layers, maximally 4 microns thick. With a bath temperature of 20.degree. to 90.degree. C. and a current density of 0.5 to 80 A/dm.sup.2, aluminum or aluminum alloys are bright-anodized. Apart from the fact that with this method, one works at higher temperatures when using high salt concentration electrolytes, these treatments ("burnishing processes") lead to shiny films which are often undesirable.
Since the dissolution of the aluminum proceeds faster than the formation of the oxide, only an aluminum oxide layer of less than 4 microns can be generated with the method described in this U.S. Patent even after an extended anode oxidizing time. In addition, because of the large amount of carbonate in the bath and its reaction with aluminum oxide as a result of absorption of carbon dioxide from the air, the oxide layer formation is limited to the so-called forming layer of less than 1 micron thickness after a short time in the bath. Under the anode oxidizing conditions given there, the removal rate is so high that for more thinly aluminized places of aluminized parts, the aluminum is stripped off instead of anodized. In particular, no close tolerances of the aluminum coating can be maintained by the method described in U.S. Pat. No. 4,188,270. Also, aluminum materials can thus not be provided with a thicker aluminum layer.
A special problem arises when forming an article from a combination of metallic materials with aluminum as a protective coating. When, for example, iron, copper, nickel and zinc and their alloys are to be coated, they will dissolve anodically in the known acid anodizing electrolytes (if poled anodically) if there are insufficiently coated material surfaces. In such materials, corrosion phenomena of so high a degree occur that aluminum-coated parts become unusable in decorative respects, as well as functionally, or are destroyed.
This fact limits the application of aluminizing in the decorative functional surface sector heavily, so that for example, stainable galvano-aluminum Eloxal.RTM. (Siemens trademark (oxidatively anodiyed) aluminum coated articles) films could be realized so far, for instance, on spectacle frames, cigaret lighter cases, writing instruments etc. only with difficulty from a technical point of view, or not at all.
For anodizing aluminized parts with insufficient coverage, which is carried out in acid anode oxidizing baths, it is necessary to provide the exposed places with a so-called masking varnish prior to the anode oxidizing. After the oxidizing, such varnishes must be removed by either stripping them off or dissolving them with a suitable solvent. In the case of cavities or holes, in which there is no aluminum, this can be aided by sealing them off by plugs or the like. While this procedure is in principle possible, it is technically demanding and uneconomical. For composite parts, for instance, such as those with hinges (spectacles), the foregoing masking procedure is unusable for decorative reasons (straining of the Eloxal.RTM. layers) and anodizing is therefore not possible in principle.
It is, therefore, an object of the invention to provide aluminum materials as well as aluminum-coated parts, especially ferrous materials, with hard, abrasion-proof and stainable, thick aluminum oxide layers especially having a thickness of 10 to 20 microns, while avoiding the foregoing disadvantages. Also, it is an object to provide a process which works even though the aluminum coating has defects. Such defects would include, for instance, the uncoated contact areas, or in the case of profiled parts, the uncoated places which may exist because of the limited throwing power o aluminizing processes. In the anodizing, thick aluminum oxide layers are to be generated also on partially aluminized, commonly used metals such as ferrous, nonferrous, nickel and zinc diecasting materials without destroying the base material.